Project Summary/Abstract: Neuroblastoma (NB) is the most common cancer of infancy and arises from immature cells of the sympathetic nervous system (SNS) that derive from neural crest stem cells (NCSCs). NB is responsible for 15% of pediatric cancer deaths with the majority of high-risk NB patients eventually relapsing with resistant disease. Finding novel treatments, especially for relapse disease, is desperately needed for high-risk neuroblastoma. The standard model of neuroblastoma initiation assumes a block in differentiation. We propose that SNS factors important for the growth and establishment of the SNS are co-opted by neuroblastoma mutations to instead support the growth and establishment of the tumor. Further based on our preliminary findings we propose a class of drugs called BET inhibitors, which were found to be very effective against neuroblastoma, partially function by suppressing important SNS factors. Here, we hypothesize that cell fate regulators, ASCL1, PHOX2B, HAND2 and GATA3 are critical for NB oncogenesis and responsible for NB sensitivity to BET inhibitors. The two aims we will pursue in this proposal are: Aim 1: To interrogate SNS lineage factors as critical targets of BET inhibition in NB and Aim 2: To determine if PHOX2B mutants, HAND2, ASCL1 and GATA3 contribute to malignant transformation of primary NCSCs. For the first aim we will determine in neuroblastoma cell lines, if BET inhibitors work through turning off critical SNS factors. Thus, here we will: 1) Interrogate the impact of BET inhibitors on SNS lineage factors expression. 2) Investigate loss of the BET protein BRD4 binding in response to the BET inhibitor JQ1, in human NB cell lines. 3) Test if silencing SNS lineage factors individually or in combination interferes with growth of human NB cell lines and NB tumors. 4) Determine if lineage factors expressed in trans from the JQ1-insensitive MSCV promoter conveys resistance to BET inhibitors in human NB cell lines and patient derived xenograft tumors. To test the second aim we will use a unique system we have developed for turning normal immature SNS mouse cells, the NCSCs, into neuroblastoma to determine if the important SNS factors can contribute to initiating neuroblastoma. For the second aim we will: 1) Determine if the SNS core factors Ascl1, Gata3, Hand2 and mutant Phox2b block NCSC differentiation into sympathetic neurons. 2) We will determine if the SNS core transcription factors promote tumorigenic behavior in vitro. 3) Determine if the SNS core transcription factors promotes the formation of NCSC-derived NB tumors alone or in combination with N-Myc or 1p36 loss of heterozygosity, two common mutations in NB. Overall, our work will inform us of whether factors important for SNS development also contribute to neuroblastoma. Further our ability to engineer neuroblastoma tumors from immature SNS mouse cells uniquely enables us to illuminate novel mechanisms of neuroblastoma sensitivity to BET inhibition and discover new genetic contributors to this disease.